1. Field of Invention
The present invention relates to a spread spectrum clock generator. More particularly, the present invention relates to a spread spectrum clock generator with frequency tuning.
2. Description of Related Art
High-speed systems, such as wireless phones, optical fiber links, microcomputers, and high-end system-on-a-chip (SOCs), reach the GHZ level. Therefore, the electronic devices need to upgrade their operating frequency in order to keep up with the speed increased systems. Furthermore, since many circuits are integrated on a chip, the clock signal is entirely distributed on the chip, and the clock skew problem arises as a result.
For example, when an input clock drives a chip, an uncertain delay appears between the input clock and the internal clock, which might make the chip work incorrectly. In order to synchronize the system clock and suppress the clock skew of the chip, the phase-locked loop (PLLs) and delay-locked loop (DLLs) have been applied in many high-speed circuits and systems.
As the transfer rates increases, the Electro-Magnetic Interference (EMI) that affects or destroys the performance of peripheral electronic components gets even worse, and the peripheral electronic components can not be used as a result. Since the clock signal generated by the clock generator is the main source causing the EMI phenomenon, it is important to reduce the EMI phenomenon caused by the clock generator.
Presently, Spread Spectrum Clock Generator (SSCG) is commonly used to reduce the EMI phenomenon. As the name suggests, “Spread spectrum” means to spread the center frequency of the clock signal with less amount, which reduces the energy of the main frequency and disperses the reduced energy to other frequencies than the main frequency. Since the energy of the main frequency is reduced, the EMI phenomenon can also be reduced.
Spectrum spreading, varying the frequency of the clock signal, is used to reduce the EMI effect. Although the spread spectrum reduces the EMI effects, the jitter of the clock signal deteriorates, such that users often need to compromise between the clock jitter and the spread amount. The amount of the spread spectrum of the clock generator cannot be adjusted flexibly, and the user cannot adjust the amount of spread spectrum in accordance with the actual situation. Because users can not trade-off between the amount of the spread spectrum and the clock jitter, the overall performance goes down.
Therefore, there is a need for a new spread spectrum clock generator which can adjust the average frequency of the clock signal more flexible.